Add Vulkan GPU enumeration support (related to #160)

[gn64]

Summary

Adds a safe Rust wrapper for Vulkan GPU enumeration that mirrors the SYCL request in #160 but targets the ggml_backend_vk_* APIs instead.

What’s included

• build.rs
  • allow-lists the following symbols when the vulkan feature is enabled
    ggml_backend_vk_get_device_count, ggml_backend_vk_get_device_description,
    ggml_backend_vk_get_device_memory, ggml_backend_vk_buffer_type
• src/backend/vulkan.rs
  • VkDeviceInfo — human-readable metadata (name, VRAM, buffer type)
  • list_devices() — returns Vec<VkDeviceInfo> in discrete-then-integrated order
• tests
  • enumerate_must_not_panic
  • sane_device_info

Why

Selecting the optimal GPU at runtime is vital on multi-GPU rigs (e.g. dGPU + iGPU).
While ggml already exposes Vulkan back-end helpers, whisper-rs could not reach them.
This PR fills that gap and lets downstream users build zero-copy buffers on the
device returned by list_devices().

Notes

• Feature-gated behind vulkan — no extra dependencies.
• Verified on Windows (msvc).

[tazz4843]

Ran local tests on my laptop (Fedora 41, Ryzen 7 7840HS iGPU) a few weeks ago and forgot to report back here (they succeeded). Thanks for the PR :)